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Fingertips and forehead are most sensitive to pain

Ouch! Our forehead and fingertips are the most sensitive to pain, suggests research that used lasers to give volunteers sharp shocks across their body.

The study was the first to look at how our ability to work out where something hurts – called “spatial acuity” – varies across the body. This hadn’t been done before because it is very difficult to activate the nerve fibres that process pain without simultaneously stimulating those that process touch, says lead author Flavia Mancini at University College London.

To overcome this hurdle, Mancini and her colleagues used two small lasers to deliver short painful stimuli to 26 volunteers. The distance between the two lasers was reduced until the volunteers couldn’t distinguish between them. The experiment was repeated across the whole body.

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The volunteers could distinguish between the two lasers more accurately at areas closest to their trunk, and less accurately towards their extremities – the opposite of what is seen when researchers test how we feel touch. For example, pain acuity was much greater at the shoulder than the wrist. The volunteers were also more sensitive to pain in areas closest to the trunk. There were two exceptions, however&colon; volunteers were most sensitive to the test on their fingertips and forehead.

This is unusual because our ability to perceive pain depends on how many nerve fibres that trigger the sensation of pain are present, says Mancini. The greater density of pain fibres you have in an area, the more accurately you perceive pain.

Localised pain

Nerve fibres are arranged differently for pain and touch. For pain, they are more dense in the areas near the trunk and decrease out towards the hands and feet, while touch fibres are most dense at our extremities. So somehow we must have learned to localise pain on the fingertips and forehead despite not having dense numbers of fibres there, says Mancini.

The team hypothesise that information from pain receptors in the fingertips and forehead is enhanced in the areas of the brain responsible for perceiving pain.

The team also performed the experiment on a man with a rare condition that destroyed his nerve fibres for touch, resulting in him having no sense of touch below his neck. He had the same spatial acuity for pain as the other participants, showing that you don’t need a functioning touch system to feel pain, says Mancini.

It is important to show that the two systems are distinct and to have a test that distinguishes between them, says Mancini, because when people have long-term chronic pain you need to identify which set of fibres are involved and to what degree.

“You can take biopsies of the skin out and count the different fibres, but this is invasive and costly and you don’t know whether the nerves are functioning or not,” she says. “This test is quick and ‘in vivo’ so you can assess the state of their touch and pain fibres very quickly and decide on what treatment is best.”